Interpretive Summary: Leaf rust is a disease of wheat caused by the fungus Puccinia triticina. Wheat cultivars in the U.S. are attacked yearly by this fungus. One type of leaf rust attacks bread wheat cultivars, which are commonly grown in the U.S. Another type of leaf rust attacks durum wheat, which is mostly grown in North Dakota, Arizona, California, Mexico, and other countries in Europe, South America, and North Africa. In 2001 durum wheats in Mexico were attacked by a new type of leaf rust. In recent years durum wheats in Europe and South America have also suffered yield losses due to increased leaf rust infections. In this study, we examined leaf rust from durum wheats in Europe, South America, Mexico, California, and North Africa, to see if these recent collections were similar for virulence to resistance genes in wheat. We found that collections of durum leaf rust from Europe, South America, Mexico, and California, were identical or very similar for virulence to rust resistance genes. This indicated that a single source of durum leaf rust may have spread from a single region to many wheat producing regions of the world.

Technical Abstract:
Collections of Puccinia triticina from durum wheat from Argentina, Chile, Ethiopia, France, Mexico, Spain and the U.S., and representative isolates of leaf rust from bread wheat from the United States, were tested for virulence phenotypes on 35 near-isogenic lines of Thatcher wheat. Virulence on Lr10, Lr14b, Lr20, Lr22a, Lr23, Lr33, Lr34, Lr41, and Lr44 was the most frequent on durum wheat. Cluster analysis showed that P. triticina from durum wheat from South America, North America and Europe had an average similarity in virulence of 90%, while isolates from Ethiopia were less than 70% similar to the other durum leaf rust isolates. Most isolates from Ethiopia were avirulent to Thatcher and all near-isogenic lines. The bread wheat isolates had an average similarity in virulence of 60% to all durum leaf rust isolates. Leaf rust from durum wheat was avirulent to many Lr genes commonly found in bread wheat. It is possible that P. triticina currently found on durum wheat world-wide had a single origin, and then spread to cultivated durum wheat in North America, South America and Europe, while P. triticina from Ethiopia evolved on landraces of durum wheat genetically distinct from the cultivated durums grown in Europe and the Americas.